Avidin and biotin, or streptavidin and biotin have an extremely high affinity (Kd=10−15 to 10−14 M). This is one of the strongest interactions between two biomolecules. At present, the interaction between avidin/streptavidin and biotin has been widely applied in the field of biochemistry, molecular biology or medicine (Green, (1975), Adv. Protein Chem., 29: 85-133; Green, (1990), Methods Enzymol., 184: 51-67). Avidin is a basic glycoprotein derived from albumen, and its isoelectric point exceeds 10. On the other hand, streptavidin is a protein derived from one type of Streptomyces (Streptomyces avidinii). Its isoelectric point is around the neutral range, and it does not comprise a sugar chain. The two types of proteins each form a tetramer, and they each bind to a molecule of biotin per subunit. Their molecular weight is approximately 60 kDa.
In recent years, a drug delivery method involving the combination of an antibody molecule with the aforementioned high binding ability of such avidin/streptavidin and biotin, namely, a pretargeting method has been conceived (Hnatowich, (1987), J. Nucl. Med., 28, 1294-1302). However, since a chicken-derived avidin or a microorganism-derived streptavidin exhibits high immunogenicity in human bodies, it has been problematic in that an anti-avidin/streptavidin antibody is generated at an early stage after administration of such avidin/streptavidin to a human body. This is a cause that prevents the practical use of a pretargeting method (Paganelli, (1991), Cancer Res., 51, 5960-5966).
In order to solve the aforementioned problem, a study paper regarding reduction in the immunogenicity of a streptavidin had been published in years past (Subramanian, (1998), Bioch. and Mol. biol. Int., 43, 357-82). However, the problem regarding the immunogenicity of a streptavidin in human bodies has not yet been solved.